JPH0570579B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for producing a crosslinked polyolefin closed cell cell.

[Conventional technology]

Conventionally, methods for producing low-density crosslinked polyolefin foams include, for example, methods for producing foams by crosslinking by irradiating ionizing radiation and then heating and foaming, or chemically crosslinking and then heating under pressure or normal pressure. Foaming manufacturing methods are well known. Among these, as an improvement on the latter method, a composition in which polyolefin is mixed with a crosslinking agent and a blowing agent is heated under pressure for a certain period of time to partially decompose the blended crosslinking agent and blowing agent to obtain an intermediate foam. The first step,
A production method has been proposed which includes a second step of further heating the intermediate foam under normal pressure for a certain period of time to decompose the crosslinking agent and blowing agent that were not decomposed in the first step (Japanese Patent Publication No. 45-29381). , Japanese Patent Publication No. 2-42649, etc.).

[Problem to be solved by the invention]

According to this method, a low-density, thick crosslinked polyolefin foam having uniform, fine closed cells can be obtained, but it is necessary to use a mold with tapered sides in the first step. This is because if a mold without a taper were used, the intermediate foam would not be removed smoothly. Therefore, in the first step, a mold with tapered sides is used, and when the pressure is removed, the expanded polymer pops out of the mold to obtain a foam, but during the popping process, it expands into a shape similar to the shape of the mold. . If this intermediate foam is heated and foamed as it is in a rectangular parallelepiped mold that is not a closed system under normal pressure, it will cause a reduction in entrainment at the tapered portion. Therefore, the tapered portion of the intermediate foam must be cut out in advance. Therefore, there are problems of poor material efficiency and poor workability. Therefore, an object of the present invention is to provide a method for producing a crosslinked polyolefin closed cell having excellent material efficiency and workability, and excellent physical properties.

[Means to solve the problem]

The method for producing a crosslinked polyolefin closed-cell foam according to the present invention is to press a crosslinkable foamable resin composition obtained by adding and kneading a crosslinking agent, a blowing agent, and a blowing aid to a polyolefin and filling it into a closed mold. , heating under pressure to obtain a cross-linked unfoamed resin composition, heating the composition between a pair of metal plates to partially decompose the blowing agent within an area limited by the metal plates; The composition is biaxially expanded to obtain an intermediate foam having a plate-like morphology, and the intermediate foam is heated to decompose the remaining blowing agent. It is characterized by consisting of the following. In the present invention, in the first step, only the crosslinking reaction is performed in an unfoamed state in order to facilitate mold release. In the subsequent foaming step, foaming is first limited to biaxial expansion and then three-dimensional expansion under normal pressure. By the method described above, an extremely uniform foam having a closed cell structure and a foaming ratio of 20 to 70 times can be obtained. The polyolefins used in the present invention include low density, medium density and high density polyethylene, ethylene vinyl acetate copolymer, poly 1,
2-Butadiene, ethylene propylene copolymers, copolymers of ethylene with up to 45% of methyl, ethyl, propyl or butyl acrylate or methacrylate, chlorination of said homopolymers or copolymers with a chlorine content of up to 60% by weight products, mixtures of two or more of the aforementioned polymers, and mixtures of the aforementioned polymers with isotactic or atactic polypropylene. The blowing agent used in the present invention is a chemical blowing agent having a decomposition temperature higher than the melting point of the polyolefin used. Such chemical blowing agents include azo compounds such as azodicarbonamide and barium azodicarboxylate, nitroso compounds such as dinitrosopentamethylenetetramine and trinitrosotrimethyltriamine, hydrazide compounds p,p'-oxybis(benzenesulfonylhydrazide) , sulfonyl semicarbazide compounds such as p,p'-oxybis(benzenesulfonyl semicarbazide) and toluolsulfonyl semicarbazide. Among these blowing agents, azodicarbonamide is preferred.
This is because azodicarbonamide has a high nitrogen content, the decomposition gas amount and decomposition temperature are easily matched, and it is commercially available at low cost. The blowing agent is usually used in an amount of 10 to 25 parts by weight per 100 parts by weight of the polyolefin, resulting in a foam with an expansion ratio of 20 to 50 times. In the present invention, the amount of decomposed gas and the decomposition temperature of the blowing agent can be changed by adding a blowing aid. Examples of such blowing aids are compounds containing urea as a main component, metal oxides such as acidified zinc and lead oxide, salicylic acid as a main component,
These include stearic acid and the like, that is, compounds containing higher fatty acids, metal compounds of higher fatty acids, and the like. The blowing aid must be selected to be compatible with the blowing agent. In the case of the present invention, crosslinking agents are used which have a lower decomposition temperature than the blowing agents used in the polyolefins. As a result, when the composition is heated, the decomposition of the crosslinking agent and, ultimately, the crosslinking reaction can proceed. Subsequent heating causes decomposition of the blowing agent. That is, the decomposition temperature of the crosslinking agent is
It must be lower than the decomposition temperature of the blowing agent or mixture of blowing agents and of the blowing aid, so that the crosslinking reaction in the closed mold can proceed reliably in the first stage without a blowing reaction. As a crosslinking agent, an organic peroxide is used which decomposes upon heating to form radicals capable of bringing about intermolecular or intramolecular crosslinking. Examples of such organic peroxides are dicumyl peroxide,
1,1-di-t-butylperoxy-3,3,5
-trimethylcyclohexane, 2,5-dimethyl-2,5-di-t-butylperoxyhexane,
These include 2,5-dimethyl-2,5-di-t-butylperoxyhexine, α,α-t-dibutylperoxydiisopropylbenzole, t-butylperoxyketone, t-butyl-oxybenzoate, and the like. The peroxide that best matches the type of polyolefin must be selected.
The crosslinking agent is preferably used in an amount of 0.3 to 1.5 parts by weight, preferably 0.5 to 1.0 parts by weight, based on 100 parts by weight of the polyolefin. For the purpose of improving the physical properties or lowering the price of the composition used in the present invention, compounding agents (fillers) that do not have a significant adverse effect on crosslinking, such as carbon black, zinc white, titanium oxide, calcium oxide, etc. Metal oxides such as magnesium and silicon oxide, carbonates such as magnesium carbonate and calcium carbonate, fiber materials such as pulp, and various dyes,
Pigments, fluorescent substances, and other commonly used rubber compounding agents can be added as necessary. The foamable crosslinkable composition is obtained by kneading the ingredients using, for example, a suitable heated mixing roll, extruder, or the like. Next, preferred foamable crosslinkable compositions according to the present invention will be described. Composition A In order to produce a foam with an expansion ratio of 30 times, 100 parts by weight of low density polyethylene, 14 to 17 parts by weight of azodicarbonamide, 0 to 0.4 parts by weight of zinc white or activated zinc white, or 0 to 0.6 parts by weight of zinc stearate. 0.5 to 0.8 parts by weight of dicumyl peroxide are kneaded. Composition B In order to produce a foam with an expansion ratio of 30 times, ethylene containing 5 to 20% by weight of vinyl acetate is used.
A mixture consisting of 100 parts by weight of vinyl acetate copolymer, 14 to 17 parts by weight of azodicarbonamide, 0 to 0.4 parts by weight of zinc white or activated zinc white, or 0 to 0.6 parts by weight of zinc stearate, and 0.5 to 1.0 parts by weight of dicumyl peroxide is mixed. You can also practice it. Composition C In order to produce a foam with an expansion ratio of 40 times: Low density polyethylene 100 parts by weight Azodicarbonamide 18-22 parts by weight Zinc white or activated zinc white 0-0.3 parts by weight Dicumyl peroxide 0.5-1.0 parts by weight Knead the mixture consisting of parts. Composition D In order to produce a foam with an expansion ratio of 40 times, 100 parts by weight of an ethylene-vinyl acetate copolymer containing 5 to 20 parts by weight of vinyl acetate, 18 to 22 parts by weight of azodicarbonamide, or A mixture consisting of 0 to 0.3 parts by weight of activated zinc white or 0 to 0.6 parts by weight of zinc stearate and 0.5 to 1.2 parts by weight of dicumyl peroxide may be kneaded. Next, the crosslinking foaming method according to the present invention will be explained in detail: In the first process step, the crosslinkable foamable resin composition obtained as described above is filled into a closed mold having a desired shape, and under pressure. Heat it up. "Under pressure" means that the crosslinkable foamable resin composition in the mold is pressed at 10 kg/cm ² by a suitable press, for example a hydraulic press.
, preferably in the range of 20 to 200 Kg/cm ² . At pressures below 10 Kg/cm ² , excessively coarse and non-uniform cells are formed in the foam. Heating is performed at a temperature sufficient to decompose the crosslinking agent. Further, the temperature may be higher than the decomposition temperature of the crosslinking agent. However, the temperature must be below that at which the blowing agent does not substantially decompose. Such a temperature range is typically between 20 and 160°C, preferably between 135°C and
In the range of 155℃. Under pressure, preferably 135~
For sufficient crosslinking in closed molds under heating in the range of 155 °C, choose a heating time of 20 to 60 minutes, depending on the size of the mold. As a result, in the first step, a crosslinked, unfoamed resin composition whose external shape corresponds to the shape of the mold is obtained. In this first process step, the gel fraction of the crosslinked, unfoamed resin composition is obtained in the range of 1 to 70%. If this gel fraction exceeds 70%, the crosslinked, unfoamed resin composition cannot be sufficiently expanded in the subsequent step. In other words, it cannot be foamed. However, when the gel fraction is less than 1%, the cell structure does not have sufficient strength,
Decomposed gas from the blowing agent escapes. Gel fraction refers to the after-extraction value for the pre-extraction sample (polyolefin composition) after extraction for 24 hours under refluxing solvent trichlorethylene (triclene) using a Soxhlet reflux extractor using a 40-50μ glass filter. Sample weight ratio (in %)
It is. The degree of crosslinking is proportional to the increase in gel fraction. Upon completion of the first process step, the crosslinked, unfoamed resin composition is released from the closed mold. The crosslinked, unfoamed resin composition can be easily removed from the mold.
This is because foaming had not yet taken place and the composition could not be pressed and stuck to the mold wall by the foaming pressure. During the performance of the crosslinking reaction in the first process step, an initial decomposition of a very small amount of blowing agent is unavoidable. Such decomposition of the blowing agent causes the crosslinked unfoamed resin composition to expand to at most twice its original volume after being taken out of the mold. However, such expansion, which is extremely small for the foaming process, is not considered foaming in the present invention. Therefore, it was expressed as a crosslinked "unfoamed" resin composition. Since the composition can be easily removed from the mold, there is no need to use a tapered mold as in the conventional method described above. The crosslinked unfoamed resin composition taken out from the mold is used as it is in the next step. There are no tapered sections that have to be removed. Next, the crosslinked unfoamed resin composition is heated between a pair of substantially parallel metal plates adjusted to a foaming temperature so that the foaming agent contained in the composition is partially decomposed. In this case, the composition expands biaxially in a plane defined between the two metal plates, and an intermediate foam having a plate-like shape corresponding to the area between the two metal plates is obtained. The necessary foaming temperature for this process step depends on the type of blowing agent used and the mixing proportions and is selected between 150 and 190°C, preferably between 160 and 180°C. In this case, the composition is gradually foamed by uniform heat transfer from the heated metal plate to the composition. The heating time of the composition in this second stage is selected between 15 and 45 minutes depending on the size and composition of the composition. The intermediate foam obtained in the second stage preferably has an expansion ratio of 6 to 10 times and has a remaining blowing agent decomposition in the range of 15 to 60% by weight. In a third process step, the intermediate foam removed from both metal plates is heated to the above-mentioned foaming temperature, preferably to a temperature above this, particularly preferably to a temperature in the range from 160 to 200°C. Heating time is preferably 20
~60 minutes, during which time the blowing agent remaining in the intermediate foam is completely decomposed and the intermediate foam is free to expand in all directions to form the final foam. In the third process step, the intermediate foam is heated and foamed in a nitrogen atmosphere or in a metal or salt bath containing a heating medium such as a rose alloy, a wood alloy, or one or more salts, such as sodium nitrate, potassium nitrate, etc. It is carried out in a salt bath containing potassium nitrite, etc. The intermediate foam is placed in a non-airtight open mold and heated together with the mold in a heating medium maintained at the appropriate foaming temperature. Preferably, the mold is equipped on its outer surface with an electrical heating device or a heating jacket through which a heating medium, such as steam, heating oil, etc., circulates. By heating the mold, the intermediate foam placed therein is heated indirectly through the walls. The intermediate foam can also be covered with one or more heated metal plates that are movable up and down. After heating the intermediate foam for a predetermined period of time and then cooling, a foam is obtained. Heating time is 10 to 50 minutes. In this way, a foam with a homogeneous microcellular structure with strong cell walls is obtained, which foam has a high expansion ratio of up to approximately 70 times the original volume, depending on the amount of blowing agent added. The size of the bubbles is in the range of 20 to 150μ, and a degree of crosslinking with a gel fraction of about 75% is obtained. Next, the present invention will be explained in detail with reference to Examples. Example 1 Low density polyethylene (MFR1.5) of 0.920g/ ^cm3 , 17% per 100 parts by weight of polyethylene
A composition containing parts by weight of azodicarbonamide, 0.1 parts by weight of zinc white, and 0.6 parts by weight of dicumyl peroxide was heated to 110 parts by weight using a mixing roll.
The mixture was thoroughly kneaded homogeneously at ℃. The obtained kneaded material,
The mixture was placed in a mold (size: 750 x 360 x 28 mm) in a press controlled at a temperature of 150°C and heated for 40 minutes under a pressure of 100 Kg/cm ² to obtain a crosslinked unfoamed resin composition. The gel fraction of the composition was 42%. The composition was then heated for 30 minutes at a temperature of 170° C. between a pair of metal plates with a spacing of 28 mm. At this time, the azodicarbonamide was partially decomposed and expanded biaxially within the space defined by both metal plates. The resulting intermediate foam was taken out from between both metal plates. The intermediate foam had expanded to 8 times its original volume and approximately 27% by weight of the azodicarbonamide had already decomposed. This intermediate foam was then placed into an open, non-hermetically closed mold with dimensions 2080 x 1060 x 95 mm. The mold has a heating jacket through which steam circulates;
Heated at 175°C for 30 minutes. After cooling, the foam was removed from the mold. It was found that the azodicarbonamide remaining after the second process step was completely decomposed and the expansion ratio was 30 times. The gel fraction was 70%. The foam produced in this way had a uniform fine cell structure with high toughness, and the cell size was about 50μ. The final foam had a heat of 90 mm and an apparent density of 0.030 g/cm ³ .
The whole was usable as a foam material and there was no waste up to this point in production. Example 2 Density 0.937/cm ³ containing 14% by weight of vinyl acetate
Ethylene-vinyl acetate copolymer (MFR1.5)
A composition containing 17 parts by weight of azodicarbonamide, 0.05 parts by weight of zinc white, and 0.8 parts by weight of dicumyl peroxide was homogeneously kneaded at 90°C using a mixing roll based on 100 parts by weight. The obtained kneaded product was filled into a mold (size: 750 x 360 x 28 mm) in a press controlled at a temperature of 145°C, sealed, and heated for 40 minutes under a pressure of 100 kg/cm ² to crosslink it. An unfoamed resin composition was formed. The crosslinked unfoamed resin composition thus obtained was then taken out from the mold. When measuring the gel fraction of this composition,
It was 35%. The composition was then prefoamed between two metal plates as described in Example 1. The foaming ratio of the obtained intermediate foam was 6 times, which was about 6 times that of azodicarbonamide.
20% had decomposed. It was then foamed in the same manner and under the same conditions as described in Example 1. The foam had a gel fraction of 75% and a foaming ratio of 30 times. This foam also has a tough, uniform fine closed cell structure, and the size of the cells is approximately
It was 50μ. Foam thickness 90mm, apparent density
It was 0.030g/ ^cm3 .

【Effect of the invention】

The method according to the invention allows the production of foams with homogeneous, fine, closed cells with relatively short working times and working cycles with high productivity. According to the present invention, the operating method is easy, high yields are obtained, and waste is reduced. The closed-cell foams made of crosslinked polyolefins obtained according to the invention can be used advantageously as upholstery materials, insulation materials and coating materials.

Claims (1)

[Claims] 1. A crosslinkable foamable resin composition obtained by adding and kneading a crosslinking agent, a blowing agent, and a blowing aid to a polyolefin is pressed and filled into a closed mold, and heated under pressure. A crosslinked unfoamed resin composition is obtained, and the composition is heated between a pair of metal plates to a foaming temperature to partially decompose the blowing agent, so that the composition is heated within an area defined by the metal plates. is biaxially expanded to obtain an intermediate foam having a plate-like shape, and the intermediate foam is heated under normal pressure to decompose the remaining foaming agent and expand in three dimensions. A method for producing a crosslinked polyolefin closed cell foam. 2 The degree of crosslinking of the crosslinked unfoamed resin composition is 1 to 70%
The method according to claim 1, wherein the gel content is in the range of . 3. The method according to claim 1 or 2, wherein the intermediate foam has a foaming ratio of 6 to 10 times. 4. The crosslinking reaction is carried out under a pressure in the range of 20 to 200 Kg/ ^cm2 ,
The foaming is carried out at temperatures up to 160°C, preferably in the range 135-155°C, and at the initial stage of foaming.
Claims 1 to 3 are carried out at a temperature in the range from 160 to 190°C, preferably from 160 to 180°C, and in a subsequent second stage at a temperature in the range from 160 to 200°C. The method described in any one of the above. 5. The method according to any one of claims 1 to 4, wherein the crosslinked unfoamed resin composition is expanded to a volume of 1 to 2 times the mold volume.